C. elegans is a classic model organism that has played key roles in the discovery of fundamental biologic processes such as caspase cell death pathways and RNA interference. However, its applications to the study of host-virus interactions have been highly limited until recently, due to the lack of viruses known to infect C. elegans. We recently discovered Orsay virus, the first and to date only virus capable of naturally infecting C. elegans, by screening wild isolates of C. elegans by high throughput sequencing. As a positive sense RNA virus distantly related to nodaviruses, Orsay infection of C. elegans provides a novel experimental platform to explore host-RNA virus interactions. This model has already provided novel insight into the role of RNA interference and the ubiquitin-proteasome pathway in antiviral immunity. While this infection model is very powerful, there are currently no known viruses capable of naturally infecting Caenorhabditis nematodes derived from viral families whose members include human pathogens. Thus, the primary goal of this project is to identify novel viruses in Caenorhabditis nematodes that are directly related to known human pathogens and to establish nematode infection models using these viruses. Such models would be the foundation for future studies (outside the scope of this R21) to define host-virus interactions that might be evolutionarily conserved. For example, if a novel herpesvirus or filovirus etc., that infects C. elegans were to be identified, screens that capitalize upon the robust genetic tractability (e.g. forward EMS screens or reverse genetic whole genome RNAi screens) of C. elegans could then be devised to define both antiviral and proviral host factors. Furthermore, the identification of viruses of different viral `lifestyles' capable of infecting Caenorhabditis nematodes would complement and augment the Orsay/Santeuil/LeBlanc system by enabling comparative analyses to determine whether a given phenotype was virus specific or more broadly pan-viral. The aims of this grant are: (1) To identify novel viruses present in a unique collection of >400 wild isolates of C. elegans and C. briggsae using high throughput sequencing; (2) To establish an experimental nematode infection system using the best candidate virus identified in Aim 1 (ideally a novel virus related to pathogenic human virus) and define the fundamental virologic parameters of this system. Successful completion of these aims will generate a completely novel platform for interrogation of host-virus interactions that should provide in the long- term novel insights into human disease and biology.